This invention relates to a signaling unit for the control of digital communications transmitted over a switching network. More specifically, it relates to a signaling unit having a variable signaling-message repertoire used in setting up and releasing communications channels in a digital switched network such as ISDN, and also in performing various maintenance functions in the network and providing enhanced subscriber services.
The invention will be described in connection with a data network in which signaling messages are transmitted over a signaling channel and data, including digitized voice signals, are transmitted over a "CIC" channel. Access signaling is governed by a standard suite of protocols known as Signaling System No. 7 (SS7) which defines the various signaling messages, including their formats, used in call-setup and release and various other control functions. A description of SS7 is set forth in Madrases, et al., An Overview of Signaling System No. 7, Proceedings of the IEEE, Vol. 80, No. 4, April 1992. While SS7 provides a fairly extensive repertoire of signaling messages, it is often desirable to augment the message set with additional messages. For example, it may be desirable to provide subscriber services that are unavailable with the standard message set.
Moreover, the requirements for additional messages will vary from one network to another and it is therefore impractical to provide each Service Switching Point (SSP) with the ability to deal with all of the possible message types. Furthermore, the requirements for additional message types at any given SSP may vary from time to time. Accordingly, it is desirable to provide a system in which the signaling-message repertoire of an SSP can be initially determined and also changed from time to time, in the field, ideally by the operator of the switching system.